The present invention relates generally to systems and methods for producing metallic powders, and more particularly to system and method for producing spherical metallic powder of uniform size and tap density by centrifugal cooling.
In industrial applications of metal and alloy powders, spherical powders which flow well and have consistently high tap density are specially desirable in powder metallurgy processes for consolidation by way of vacuum hot pressing or hot isostatic pressing at high pressure to pressed parts with near net product shape. The density of the finished part, however, is further dependent upon particle density and porosity. Further, uniformity of size and shape of powder particles beneficially affects flow and compaction characteristics of the powder. Optimizing particle density and porosity along with controlling uniformity of particle size and shape is therefore critical in obtaining uniformly high tap densities in the powder product, and in obtaining optimum and predictable physical properties and dimensional reproducibility in a finished part.
Conventional methods for producing metallic powder include chemical methods wherein powder is produced by chemical decomposition of a metal compound, mechanical methods wherein the metallic form is mechanically comminuted to the desired particle size, and physical methods wherein a molten stream of a metal or alloy is atomized by impact with a fluid, usually gas, jet. Atomization processes are commonly used in producing metallic powders, and are the most convenient for producing alloy powders of the type required for modern high temperature applications. Such an atomization process is generally a two step process comprising providing a melt of the metal or alloy, followed by disintegrating a molten stream of the melt into droplets by impact with one or more high pressure fluid streams. Powders in the size range of from about 0.1 to about 1000 microns may be produced. In the production of rapidly solidified metallic powder utilizing gas atomization techniques, small particles solidify faster and often into a different microstructure than large particles; accordingly, microstructural uniformity in finished powder compacts requires close control of particle size in limited size ranges. Atomization processes may be applicable to the production of powders of most metals of interest including iron, tin, nickel, copper, aluminum, titanium, tungsten, molybdenum, tantalum, niobium and magnesium and alloys including stainless steels, bronze, brass and nickel/cobalt based superalloys. A comprehensive survey of conventional atomization techniques is presented in "Production of Rapidly Solidified Metals and Alloys", by S. J. Savage and F.H. Froes, J Metals 36:4, 20-33 (April 1984).
Existing gas atomization processes often produce powder which is not uniformly spherical, resulting from shortcomings in the processes allowing powder particles to collide with walls or other elements of the atomization equipment before the particles solidify and cool completely. The collisions result in irregularly shaped particles exhibiting poor powder flow and nonuniform tap density. Contamination of powder particles usually also results in part from erosion of impacted equipment surfaces, which contamination deleteriously affects fatigue resistance of a finished compacted part. Prior measures to avoid this problem have included building the atomization units large enough for particles to solidify before reaching a wall or other surface within the process equipment.
The invention solves or substantially reduces in critical importance the aforesaid problems with existing atomization processes for producing metallic powder. System and method are described for centrifugally cooling metallic powder as it is formed in an atomization process. In the method described, a stream of molten metal or alloy is atomized by impact with high pressure fluid to disintegrate the stream to droplets. The droplets are cooled by passage through a chamber into which coolant fluid is injected through a plurality of jets directed through the chamber walls at a predetermined angle, which results in a swirling motion of the fluid within the chamber and causes the metallic droplets to fall within the chamber in a helical path of controllable radius. Contact of the droplets with the chamber walls during cooling and solidification is thereby avoided. The powder product is uniformly spherical in shape, uniform in size and free of contamination. Chamber size may be kept substantially smaller than with previously known powder production processes. Suitable control of the process parameters of the invention may also allow separation by size of powder product and removal of high and low density occasional contaminants. The invention is applicable to the production of a large variety of metallic powders including the metals and alloys mentioned above.
It is therefore a principal object of the invention to provide improved rapid solidification method and system for producing spherical metallic powder.
It is another object of the invention to provide method and system for producing contamination free metallic powder.
It is a further object of the invention to provide method and system for producing metallic powder of uniform size and tap density.
These and other objects of the invention will become apparent as the description of representative embodiments proceeds.